MONTE-CARLO SIMULATION FOR BACKSCATTER OF OUTGASSING MOLECULES FROM SIMPLE SPACECRAFT SURFACES

A computer program has been developed for the Monte Carlo simulation o f molecular flux impingement on simple spacecraft surfaces due to self - and ambient scattering of outgassing molecules. The outgassing and a mbient molecules are assumed to be elastic spheres with a Maxwellian v elocity distribution. Binary elastic collision processes have been ado pted in a hard-sphere collision model. A test-particle Monte Carlo met hodology is used to simulate sequentially the motions, collisions, and trajectories of the outgassing molecules inside a control volume by m eans of uniformly distributed random numbers. A unique Monte Carlo com putational scheme has been developed that reduces the computer process ing time by three to four orders of magnitude for a typical outgassing problem without introducing additional assumptions or compromising th e solution accuracy. Monte Carlo results are obtained in the form of b ackscatter flux distribution and return flux ratios, for various sampl e sizes, control-volume sizes, and outgassing and ambient flow conditi ons. The CPU time for a typical run is about 2 to 4 min on a VAX-8700 computer, and less than about 2 to 4 s on a Cray X-MP system.